M. Donnangelo, Emerson, Houston, Texas; and T. DWYER, Emerson, Dallas, Texas
Steam distribution systems are a vital part of process manufacturing because steam is a versatile means of delivering carefully controlled heat energy, usually via a heat exchanger. These exchangers can be shell-and-tube, plate, jacketed reactor or other configurations, and in all cases their purpose is to raise the temperature of a process fluid, either liquid or gas.
Using steam for such purposes avoids the need for numerous small combustion or electric heaters attached to those heat exchangers. Using steam allows the fire, or other heat source, to generate steam in a central location away from explosive environments and distribute it wherever it is needed. Some of the largest applications, such as a main distillation column, may be supported by an internal heat source, such as a fired heater, but the steam distribution system supplies many smaller installations.
Nonetheless, generating steam is very expensive, and is generally one of the highest-cost utilities in a facility. The fuel burned also creates greenhouse gases, so there is a double incentive to make the operation as efficient as possible.
Heat exchangers in a petrochemical plant. If a heat exchanger is sized correctly, the flowrate and temperature change of the process fluid will be matched by a corresponding flow of saturated steam—also balancing temperature and flow—so that the steam condenses as it transfers heat to the process fluid (FIG. 1).
Saturated steam is normally used for heat exchangers, and once it transfers its latent heat to the process, it condenses back to water. If the heat exchanger is sized correctly and the flows are correct, live steam should not normally exit the heat exchanger.
Once steam has condensed, it must be removed from the steam piping. Normally, it is returned to the boiler via a condensate-return system and added to the boiler feedwater. Steam traps are used to separate condensate from live steam (FIG. 2). How well steam traps perform affects the efficiency and sustainability of an entire steam distribution system. Steam trap service is harsh, so eventually, steam traps often fail and must be checked frequently to ensure correct operation.
Studies suggest that the normal life expectancy for a steam trap is 4 yr−8 yr, depending on the application. In a large-scale process plant, it is common for 25% of all installed units to fail in a given year. A large-scale chemical manufacturer can have several thousand steam traps, so if steam trap maintenance is not kept up to date, the negative effects can be huge.
Steam traps behaving badly. Steam traps are placed strategically throughout steam distribution systems. They may be associated with a specific heat exchanger or another piece of equipment, or they may be at a low point in the piping where condensate tends to accumulate due to gravity. Whatever the case, steam traps should release water without losing live steam. They must open to downstream pressure, while maintaining steam-line pressure. Steam system designs assume all traps are functioning normally all the time, but they can malfunction in two ways (FIG. 3).
First, some steam traps do not seal completely or can fail to open, allowing condensate to sputter out and release live steam. A U.S. Department of Energy (DOE) survey found that 5%−20% of steam traps fail annually, with the steam lost through open traps equal to 33%−50% of line capacity, which is very costly. Injecting live steam into a closed condensate capture system can be problematic if it cannot handle the pressure. Pressurizing the outlet side of a steam trap can cause it to malfunction and not release condensate as it should, multiplying the effects to all the steam traps connected to the header.
Second, the opposite can also happen where some steam traps get stuck closed. Condensate then accumulates and backs up into the steam line (FIG. 3). If it is attached to a heat exchanger, it will eventually fill with condensate and block steam flow, which can lead to heat exchanger stalls. Slugs in the steam pipes can also cause water hammer, leading to pipe corrosion, erosion and cracks.
Effects on sustainability. The two conditions mentioned above affect sustainability adversely but in different ways. When it occurs due to a failed-open steam trap, steam loss is a link in a direct energy loss chain. The boiler must work harder to compensate for the loss, so it consumes more fuel and creates more emissions, since most boilers burn oil or natural gas. Leaking steam can also lead to a shortage and system pressure sag, requiring additional boilers and leading to even more emissions. Discussions of steam leaks from steam traps and elsewhere in the system usually point to the alarming costs of additional fuel required and the growing carbon footprint to compensate for the losses.
The problems caused by failed-closed steam traps are more subtle. Discussions around this topic tend to emphasize hazards caused by slugs of water shooting through the steam system and damaging equipment. Concurrently, if the steam trap supporting a heat exchanger is stuck closed due to malfunction or over-pressurization of the condensate recovery system, steam cannot flow through the heat exchanger sufficiently to deliver the heating step that the process needs. Some steam might be flowing, but condensate backed into the heat exchanger reduces the working exchange surface area. Whatever the situation, the process fluid is not being heated sufficiently. Operators observing the effect in the control room may assume it is caused by fouling because the results are similar (FIG. 4).
Each issue invariably reduces product quality, adds cost, and increases carbon footprint. Maintenance may even call for a shutdown to check the heat exchanger for fouling, when the actual cause is much simpler and easier to fix without a shutdown.
Maintenance problems can be addressed quickly since steam traps are simple to service and new units are inexpensive. Some plants perform monitoring manually, sending technicians on rounds to determine which units seem to be performing correctly. Technicians often carry a thermal imaging device to verify the correct temperature profile or a portable audio sensor to capture characteristic sounds. These approaches can be very effective but require a human technician with sufficient training to interpret the results.
However, given personnel constraints at process plants, how often can a steam trap be inspected? Given general performance statistics, inspections several times yearly for each steam trap are a practical impossibility in most plant environments.
Automated, continuous monitoring. To solve manual monitoring problems, acoustic sensors can be mounted permanently on the piping adjacent to a steam trap, enabling continuous operational monitoring (FIG. 5). Such sensors are internally powered and use WirelessHART networks to send data to maintenance and reliability departments for analysis. These devices can be mounted with hose clamps with no process penetration, so there is no need for a shutdown.
Since monitoring every steam trap is impractical, maintenance and reliability teams should identify the most critical points and bad actors. Critical steam traps typically include those used in high-pressure and high-quality steam systems, and those associated with highly-sensitive equipment. These are normally the first to be outfitted with acoustic monitors. Once savings from improved performance begin to accumulate, new possibilities emerge. Investment in additional sensors quickly pays for itself.
Data collection and analysis software tracks individual steam traps and presents a picture of performance in real-time via preconfigured dashboards. Technicians can see which steam traps work correctly and which are in failure mode. The software estimates lost energy and resulting costs, including the effect on carbon footprint. Maintenance can plan activities appropriately, dealing with small problems before they become serious.
Naturally, the data may require interpretation. A steam trap may be reported as inoperative because the equipment only operates intermittently, or it may simply be shut off. On the other hand, a steam trap attached to a process that runs continuously, or at least regularly, should develop characteristic discharge patterns. If these indicators change, such as a sudden increase in condensate volume, there is probably an issue with the steam trap.
Basic operation vs. system improvement. Malfunctioning steam traps can make a steam system less efficient, but even when operating correctly, steam system operation can be improved. Most steam distribution systems are under-instrumented, so it is difficult to determine where energy is lost, either through leaks or heat dissipation in the piping, or where specific applications consume more steam than necessary.
Moving beyond steam trap performance monitoring is the next step if a facility is serious about sustainability. Maintaining high-quality steam throughout a distribution system requires additional attention, which is difficult without proper measurement. Maintaining steam delivery within a facility without losses from leaks is a major challenge, particularly with aging infrastructure. Measuring steam system temperature, pressure, and flowrates at strategic points delivers insights into plant performance, and technicians can use this information to perform adjustments and pinpoint issues.
Recent improvements in flowmetering technologies (FIG. 6) deliver reliable information, even in the most challenging installations. Tying flow and other variable measurements into supervisory software delivers valuable, concise information to simplify steam system operations.
Making a facility more sustainable requires paying attention to a wider range of operating parameters than may have been the case previously. Small energy losses might have been tolerated in years past, rationalizing the situation by saying, “It is not worth fixing.” Such situations are becoming more difficult to ignore because costs can escalate quickly when nothing is done, and steam traps and systems are a prime example. Adding monitoring capabilities is a critical first step, but they only identify that a problem exists. Solving the problem means acting on the information by fixing steam traps and related systems before they contribute further to ongoing waste. This is the path to sustainability. HP
NOTES
a Emerson’s Yarway Float and Thermoplastic product family
b Emerson’s Rosemount 708 Wireless Acoustic Transmitter
c Emerson’s Rosemount 8800 Series Vortex Flow Meter
MARCIO DONNANGELO is a Global Business Development Manager with Emerson in Houston, Texas, specializing in wireless measurement instrumentation technologies. He is an industrial electrical engineer with 25 yr of field experience applying industrial automation solutions for customers across multiple business segments and industries, including refining, petrochemical, chemical, pulp and paper.
Tim Dwyer is a Business Development Manager for Steam and Industrial Regulators with Emerson. He covers pressure control devices and steam equipment. Dwyer has been with Emerson for 8 years, and he has held multiple roles that have given experience with many of the various valves found in process industries. He is involved in the FCI standards committees for various steam valves used in industry and ABMA. Dwyer earned a BS degree in mechanical engineering from Iowa State University and is a Certified Energy Manager.